1. Field of the Invention
The present invention relates to the recycling of inert shielding gases used in a plasma furnace, and, in particular, to a system and process for conditioning and recycling inert gases such as argon and helium to a plasma furnace.
2. Description of the Prior Art
In many metallurgical applications, metals are melted, annealed and refined in furnaces under high heat for producing various products. One commonly-used furnace is a plasma furnace, which uses a plasma torch as a heat source, such as described in U.S. Pat. No. 3,194,741 to Baird. In general, a plasma torch is a stream of gas through which an electric arc is drawn. This heats the gas, which then can be used for cutting, welding or other purposes, such as melting the metal feedstock.
In order to control the arc and provide the proper atmosphere in the furnace, inert gases are used. These inert gases lessen problem oxidation, which occurs if oxygen or air contacts the molten metal. Besides the inert gas flowing through the torch, additional streams are used for cooling and for purging viewports and some chambers. In most applications, the entire furnace is a closed unit from which air is evacuated and then backfilled with the inert gas.
The inert gas is typically argon, helium, nitrogen or hydrogen or a mixture of these gases. The choice of gases to be used depends on the process and the particular metal to be melted. Inert gas flow rates depend on the size of the furnace, but are in the range of 40 to 150 standard cubic feet per minute for a typical-size furnace. Based on these flow rates, the cost to put nonrecycled gas through the plasma torch can be approximately $300 per hour, or $600,000 per year based upon a process time of 2000 hours per year. Thus, the desirability of gas recycle is apparent.
When the inert gas emerges from the plasma furnace, it if often at relatively high temperatures and at relatively low pressures. In addition, the output gas is typically contaminated with condensed metal and salt dusts, and other undesirable substances which must be removed. Therefore, the inert gas must be cleaned and conditioned before it may be returned to the furnace. This conditioning process includes compressing the gas to a proper input level.
Although systems for cleaning gases are old, many of the techniques which are used in general applications will not work in metallurgical applications due to the purity of the gas required for proper process conditions. The types and amount of contaminants in the gas must be reduced to very low levels in order to assure the quality of the product. For example, when air-contaminated gas is exposed to molten titanium, the titanium readily absorbs both the oxygen and the nitrogen in the air. The amount of air in the inert gas must generally be less than 100 ppm for acceptable process conditions.
In the past, inert gases in plasma furnaces have either been vented to the atmosphere or recycled using oil-less compressors. However, the life of these compressors is short, the maintenance costs are very high and the contamination level due to air infiltration through the seals is unacceptable.
Some prior systems for cleaning gas for metallurgical processers have relied on oil-filled compressors. Oil-filled screw compressors, although quite efficient, have not been widely utilized, due to the amount of oil which can be vaporized into the gas stream.